Currently, a door closer is usually mounted between a door body and a doorframe in an office or some public sites. When the door is opened, it will close automatically due to an elastic return force of the door closer, whereby to ensure accurate and timely closing to an initial position after the door is opened and providing great convenience to people's life.
Door closers are roughly classified into hydraulic door closers and air control-type door closers according to the driving sources. Hydraulic door closers have relatively complicated internal structures and therefore are not convenient for spread and application. In the prior art, air control-type door closers are mostly used generally.
In the prior art, some people make specific improvements to the air control-type door closer, e.g., the patent for invention No. 200720181355.0 entitled “Constant Speed Device for Door Closers”, wherein the door closer can reduce a return speed of the telescope arm, i.e., lessen the return initial speed of the door body thereby achieving the effect of closing the door at a constant speed in the whole stroke. However, air vents are provided on the wall of the cylinder of the patented invention, air constantly enters from outside. Since air entrains dusts, after a large amount of dusts is mixed to lubricant oil between the cylinder walls, the mixed lubricant oil with dusts are extremely liable to obstructions of through holes of the cylinder wall and deprives the door closer of normal operations. In addition, due to presence of a lot of dusts in the lubricant oil, the lubricating effect of the lubricant oil deteriorates and the lubricating capacity is weakened, the frictional force thereof is increased and the service life of the door closer is shortened.